Of all passive electrical elements (resistors, capacitors, and inductors), inductors are the most difficult to form in integrated circuits. Inductors present difficulties because they require relatively large areas and/or volumes to achieve useful values of inductance. In addition, the traditional three-dimension character of inductors is difficult to realize within integrated circuits that are essentially two-dimensional in character.
The difficulties of making satisfactory inductors of useful values that at integrable with integrated circuits are aggravated at higher frequencies. Because of the limited volumes available for components, i.e., active and passive elements in essentially two-dimensional integrated circuits, the conventional inductive structures suffer from increasing parasitic capacitances at increasing frequencies. These undesirable capacitances are produced by conductor cross overs, closely spaced wiring paths, and the like. The parasitic capacitances adversely affect and limit the frequency at which the circuits perform properly. At high enough frequencies, the effects of the parasitic capacitances can be so strong that they overwhelm the inductive characteristic of an inductance element. In more complex structures, such as transformers, employing several windings, the difficulties associated with controlling the area and volume of an element while achieving desired performance and limiting parasitic capacitances are even greater.